Regular expressions HOWTO:https://docs.python.org/3.4/howto/regex.html
For … science underlying regular expressions (deterministic and non-deterministic finite automata), you can refer to almost any textbook on writing compilers.

Metacharacters are not active inside classes: … '$' is usually a metacharacter, but inside a character class it’s stripped of its special nature.

Perhaps the most important metacharacter is the backslash, \.

Some of the special sequences beginning with '\' represent predefined sets of characters

* doesn’t match the literal character *; instead, it specifies that the previous character can be matched zero or more times, instead of exactly once.